type_complexity: ignore opaque impl Trait bounds

clippy_lints/src/types/type_complexity.rs

@@ -3,13 +3,17 @@ use rustc_abi::ExternAbi;
 use rustc_hir::intravisit::{InferKind, Visitor, VisitorExt as _, walk_ty};
 use rustc_hir::{self as hir, AmbigArg, GenericParamKind, TyKind};
 use rustc_lint::LateContext;
-use rustc_span::Span;
+use rustc_span::{Span, sym};
 
 use super::TYPE_COMPLEXITY;
 
 pub(super) fn check(cx: &LateContext<'_>, ty: &hir::Ty<'_>, type_complexity_threshold: u64) -> bool {
     let score = {
-        let mut visitor = TypeComplexityVisitor { score: 0, nest: 1 };
+        let mut visitor = TypeComplexityVisitor {
+            score: 0,
+            nest: 1,
+            type_alias_impl_trait_enabled: cx.tcx.features().enabled(sym::type_alias_impl_trait),
+        };
         visitor.visit_ty_unambig(ty);
         visitor.score
     };
@@ -33,6 +37,8 @@ struct TypeComplexityVisitor {
     score: u64,
     /// current nesting level
     nest: u64,
+    /// whether opaque `impl Trait` types can be named through `type_alias_impl_trait`
+    type_alias_impl_trait_enabled: bool,
 }
 
 impl<'tcx> Visitor<'tcx> for TypeComplexityVisitor {
@@ -42,6 +48,10 @@ impl<'tcx> Visitor<'tcx> for TypeComplexityVisitor {
     }
 
     fn visit_ty(&mut self, ty: &'tcx hir::Ty<'_, AmbigArg>) {
+        // Opaque `impl Trait` types cannot be extracted into type aliases on stable. Skip
+        // their subtree only when `type_alias_impl_trait` is not enabled for this crate.
+        let skip_nested_type = matches!(ty.kind, TyKind::OpaqueDef(..)) && !self.type_alias_impl_trait_enabled;
+
         let (add_score, sub_nest) = match ty.kind {
             // &x and *x have only small overhead; don't mess with nesting level
             TyKind::Ptr(..) | TyKind::Ref(..) => (1, 0),
@@ -72,7 +82,9 @@ impl<'tcx> Visitor<'tcx> for TypeComplexityVisitor {
         };
         self.score += add_score;
         self.nest += sub_nest;
-        walk_ty(self, ty);
+        if !skip_nested_type {
+            walk_ty(self, ty);
+        }
         self.nest -= sub_nest;
     }
 }

tests/ui/type_complexity.rs

@@ -86,4 +86,28 @@ struct D {
     ),
 }
 
+// Should not warn, because factoring `impl Trait` into a type alias is not stable (#17195).
+fn issue_17195<I, J>(
+    left: I,
+    right: J,
+) -> std::iter::Map<std::iter::Zip<I::IntoIter, J::IntoIter>, impl FnMut((I::Item, I::Item)) -> [I::Item; 2]>
+where
+    I: IntoIterator,
+    J: IntoIterator<Item = I::Item>,
+{
+    left.into_iter().zip(right).map(<[I::Item; 2]>::from)
+}
+
+// Complexity inside an opaque type cannot be factored into a type alias either.
+fn complex_opaque_bound() -> impl Fn(Vec<Vec<Box<(u32, u32, u32, u32)>>>) {
+    |_| {}
+}
+
+// The presence of an opaque type must not hide complexity in a sibling type that can be factored
+// out.
+fn complex_after_opaque() -> (impl Iterator<Item = u32>, Vec<Vec<Box<(u32, u32, u32, u32)>>>) {
+    //~^ ERROR: very complex type used. Consider factoring parts into `type` definitions
+    (std::iter::empty(), vec![])
+}
+
 fn main() {}

tests/ui/type_complexity.stderr

@@ -91,5 +91,11 @@ error: very complex type used. Consider factoring parts into `type` definitions
 LL |     let _y: Vec<Vec<Box<(u32, u32, u32, u32)>>> = vec![];
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 15 previous errors
+error: very complex type used. Consider factoring parts into `type` definitions
+  --> tests/ui/type_complexity.rs:108:30
+   |
+LL | fn complex_after_opaque() -> (impl Iterator<Item = u32>, Vec<Vec<Box<(u32, u32, u32, u32)>>>) {
+   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 16 previous errors
 

tests/ui/type_complexity_type_alias_impl_trait.rs

@@ -0,0 +1,9 @@
+#![feature(type_alias_impl_trait)]
+#![warn(clippy::type_complexity)]
+
+fn complex_opaque_bound() -> impl Fn(Vec<Vec<Box<(u32, u32, u32, u32)>>>) {
+    //~^ ERROR: very complex type used. Consider factoring parts into `type` definitions
+    |_| {}
+}
+
+fn main() {}

tests/ui/type_complexity_type_alias_impl_trait.stderr

@@ -0,0 +1,11 @@
+error: very complex type used. Consider factoring parts into `type` definitions
+  --> tests/ui/type_complexity_type_alias_impl_trait.rs:4:30
+   |
+LL | fn complex_opaque_bound() -> impl Fn(Vec<Vec<Box<(u32, u32, u32, u32)>>>) {
+   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::type-complexity` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::type_complexity)]`
+
+error: aborting due to 1 previous error
+